1. Field of the Invention
The present invention relates to a signal processing circuit for use in a wireless IC device to perform non-contact communications using, for example, near-electromagnetic field, such as an RFID, and an antenna apparatus.
2. Description of the Related Art
Hiroshi Karibe, “Extremely Understandable Book on Non-contact IC Card (provisional English title),” Nikkan Kogyo Shimbun Publishing, 2008/04/20, page 89 (hereinafter referred to as “Karibe”), discloses a reader for a non-contact IC card used as an RFID.
FIG. 1 shows an equivalent circuit of the reader for a non-contact IC card disclosed in Karibe. This non-contact IC card is a reader that communicates with non-contact IC cards in a non-contact manner. An antenna resonant circuit AR composed of an antenna coil AL, equivalent resistors R11 and R12 corresponding to the Q value of the antenna coil, and a capacitor CO is formed in a manner corresponding to a wireless IC 11.
Capacitors C1 and C2 and inductors L1 and L2 are provided in order to match impedances between the wireless IC 11 and the antenna resonant circuit AR. A diode bridge DB is provided as a detector circuit, and capacitors C31, C32, C41, and C42 are provided as a smoothing circuit.
Adjustment of the capacitor CO within the antenna resonant circuit AR allows adjustment of the resonant frequency of the antenna resonant circuit AR, ensuring a favorable communication state.
However, in such a wireless IC device for performing non-contact communications using near-electromagnetic field, variations in the inductance of the antenna coil or in the capacitance of the capacitor change the resonant frequency of the antenna resonant circuit. Further, depending on the ambient environment of the apparatus in which the wireless IC device (in particular, the antenna coil) is incorporated, the resonant frequency of the antenna resonant circuit deviates from the specified value. For this reason, there has been a need to form the capacitor CO shown in FIG. 1 using a trimmer capacitor so as to adjust the resonant frequency of the antenna resonant circuit to the specified value for each device.
For example, in the case where this wireless IC device is used in mobile phone terminals, an antenna apparatus or wireless IC device must be prepared for each of a plurality of extremely diverse devices. In the case where adjustment work is required for each apparatus, a large number of man-hours are unfavorably required, resulting in an increase in cost.
Further, the balanced terminals of the wireless IC 11 require the inductors L1 and L2, respectively. Furthermore, the two inductors, L1 and L2, are required to have both a low direct-current resistance and a high direct-current superimposition characteristic (the allowable value of the direct current). This prevents downsizing of the device. For example, a current of several hundred mA or more passes through the inductors L1 and L2. For this reason, the inductors L1 and L2 are formed using coils that are much larger than other constituent elements, thus preventing downsizing of the device.
The above-mentioned problems apply to apparatuses including a wireless IC, as well as apparatuses including a high-frequency circuit having balanced terminals. The same goes for apparatuses including an antenna resonant circuit, as well as apparatuses including a high-frequency circuit having balanced terminals and a resonant circuit.